Phase annulus to carry out a positive phase contrast

ABSTRACT

The invention relates to a phase annulus consisting of a phase shifting thin-film system, which, in the case of a cemented system, has the following structure: (first) substrate-Ag-n H -cement-(second) substrate, or a structure: (first) substrate-n 1 -Ag-n 2 -cement-(second) substrate. In the case of an uncemented structure, the thin-film system has the following structure: substrate-Ag-n H -air, wherein n H , n 1  or n 2  represent the respective refractive index of a dielectric film. Phase shifting of the positive phase contrast can be regulated in a targeted manner.

BACKGROUND OF THE INVENTION

The invention concerns a phase annulus to produce positive phasecontrast. Phase contrast microscopy has been known since Zernike.Accomplishment of this process requires production of “phase rings” byvapor-deposition of one or more thin layers on a substrate, which may beof optical glass, for instance. By skillful selection of the layerthickness, the phase of the direct light, which passes through thevapor-deposited phase, can be delayed relative to the phase of thediffracted light, which is the light that does not pass through thephase annulus, by, for instance, π/2; or it can be advanced by π/2. Thatlatter case is “positive” phase contrast.

BRIEF SUMMARY OF THE INVENTION

The objective of this invention is to make available a phase annuluswith which one can produce positive phase contrast with essentiallyconstant phase shift and a discrete transmission curve. Another part ofthe object is to establish each positive phase shift firmly within acertain phase angle interval.

BRIEF DESCRIPTION OF THE DRAWINGS

The objective is attained according to the invention through thefeatures of three example embodiments. Other advantageous embodimentsappear from the specification. See the following figures for a moredetailed explanation. They show:

FIG. 1a: A schematic representation of a cemented phase annulusaccording to a first embodiment.

FIG. 1b: A reduced plan view of what is shown in FIG. 1a.

FIG. 2: A schematic representation of the layer structure in the case ofan uncemented thin-layer system according to a second embodiment.

FIG. 3: A schematic representation of the layer structure of a cementedphase annulus according to a third embodiment.

FIG. 4a: A graph of the phase shift of a cemented phase annulusaccording to FIG. 1a.

FIG. 4b: A graph of the transmission versus the wavelength λ for acemented phase annulus having a silver layer 35 nm thick.

FIG. 5a: A graph of the phase shift versus the wavelength λ for anuncemented phase annulus.

FIG. 5b: A graph of the transmission versus the wavelength λ for anuncemented phase annulus with a silver layer thickness of 34.5 nm.

FIG. 6a: A graph of the phase shift versus the wavelength λ for a cementfor which n_(Cement)>n₁.

FIG. 6b: A graph of the transmission versus the wavelength λ for athin-layer system according to FIG. 6a.

FIG. 7a: A graph of the phase shift versus the wavelength λ for a cementfor which n_(Cement)<n₁.

FIG. 7b: A graph of the transmission T versus the wavelength λ for thethin-layer system already mentioned in FIG. 7a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a shows schematically a cemented phase annulus according to theinvention. A ring of silver (Ag) is vapor-deposited onto a substrate,for instance, a glass plate or a glass lens, and a dielectric layer withthe refractive index n_(H) is deposited on that. Inside the phaseannulus so produced, and on the entire substrate outside this phaseannulus is the cement matrix, which produces a solid body bond to thesecond substrate. It is part of the principle of the phase contrastprocess that direct light, which in FIG. 1a is incident from below onthe substrate carrying the phase annulus, passes through the phaseannulus and in so doing experiences a definite phase shift, whilediffracted light passes through all the other regions of the substratenot covered by the vapor-deposited phase annulus.

FIG. 1b shows a reduced plan view of what is shown in FIG. 1a, once moreshowing the two regions (direct light/diffracted light). FIG. 2 shows anuncemented embodiment. The plan view is similar to that of FIG. 1b. Anoptical glass or other transparent medium, e. g., crystal or plastic, isused as the substrate material. The layer system has layers of silverand a dielectric material, such as a titanium-oxygen compound,vapor-deposited in vacuum. Furthermore, it is also possible to providethe layer system with a layer that reduces reflection. Application ofthese structures is known, for example, from German Patent 2 261 780. AMenzel three-slit interferometer is used to measure the actual phaseshift.

FIG. 3 is another embodiment of a cemented thin-layer system. One cansee that a layer with the refractive index n₁ is first coated on thelower (first) substrate, followed by the actual silver layer and then bya layer with refractive index n₂. A plan view would again, purelyschematically, be that shown in FIG. 1b.

FIG. 4a shows a graph of the phase shift of a cemented phase annulusversus the wavelength λ. One can see that the four measurements in therange between 425 nm and 600 nm are between 50° and 55°. Thetransmissions found for this example (the thickness of the silver layerin this case, d_(Ag), is 35 nm) are shown in FIG. 4b. As can be seen,the transmission is 38% for a λ of 425 nm and 15% for a λ of 600 nm.This is a thin-layer system with the layer sequence:

 Glass-n₁-Ag-n₂-Cement-Glass.

The transmission of the entire system is determined by the thickness ofthe silver layer. That is linked with a phase shift. The layer withrefractive index n₂ [sic] primarily protects the silver. It comprises,for example, an oxide (Al₂O₃) of suitable thickness, e. g., 5 nm. Thedesired phase shift φ is established primarily with the n₁ layer. Therelations $\frac{\phi}{360{^\circ}} = \frac{\Delta \quad s}{\lambda}$

and

Δs=(n _(Cement) −n ₁)·d ₁

apply.

Surprisingly, it has now turned out, according to the invention, that ifa cement is used for which

n_(Cement)>n₁

for instance, with cements having n_(Cement) =1.56 to 1.59, with n₁=1.38, the resulting phase shift increases with increasing thickness d₁of the n₁ layer.

FIG. 5a shows graphically how the phase shift depends on the wavelengthλ for an uncemented phase annulus as that in FIG. 2. The phase shift isbetween 49° and 55° in the wavelength range from 425 nm to 600 mn.Similarly, FIG. 5b shows the transmission of this uncemented phaseannulus versus the wavelength λ for a silver layer with thicknessd_(Ag)=34.5 nm.

FIG. 6a shows a graph of the phase shift versus the wavelength for acement having a refractive index n_(Cement)>n₁. As can be seen, thephase shift is 59° at a wavelength of 437 nm, 71° at 482 nm, 64° at 546nm, and 72° at 585 nm. Similarly, FIG. 6b shows the transmission Tversus the wavelength λ. The transmission is 28% at a wavelength of 425nm and about 12% at 600 nm.

FIG. 7a shows the phase shift versus the wavelength λ for a cement witha refractive index n_(Cement)<n₁. The phase shift is 30° at a wavelengthof 437 nm, 40° at 482 nm, 42° at 546 nm, and 46° at 585 nm. FIG. 7b,correspondingly, shows the transmission T versus the wavelength λ. Thetransmission is 40% at 425 nm and 20% at 600 nm.

With this invention, therefore, it becomes possible to attain positivephase contrast with an essentially constant phase shift of 50° to 55° inthe wavelength range from 425 nm to 600 nm. At the same time atransmission, T, of about 35% (at 425 nm) to 15-20% (at 600 nm) isattained. That applies for both cemented and uncemented systems.Furthermore, this invention allows establishing any positive phase shiftbetween 40° and 60°.

What is claimed is:
 1. A phase annulus for producing a positive phaseshift comprising a cemented phase-shifting thin-layer system includingfirst and second substrates, a silver layer applied directly to saidfirst substrate, a layer having a refractive index n_(H) applieddirectly to said silver layer, and a transparent cement fillingintermediate space between said first and second substrates such that atransparent cement matrix remains between said layer having a refractiveindex n_(H) and said second substrate.
 2. The phase annulus according toclaim 1, wherein said first and second substrates comprise transparentmedia.
 3. The phase annulus according to claim 2, wherein saidtransparent media are chosen from the group consisting of optical glass,plastic, and crystal.
 4. The phase annulus according to claim 1, whereinsaid layer having a refractive index n_(H) comprises a dielectricmaterial.
 5. The phase annulus according to claim 4, wherein saiddielectric material is a titanium-oxygen compound.
 6. The phase annulusaccording to claim 1, wherein said thin-layer system further includes areflection-reducing coating.
 7. The phase annulus according to claim 1,wherein the phase shift produced can be established deliberately.
 8. Aphase annulus for producing a positive phase shift comprising a cementedphase-shifting thin-layer system including a substrate, a silver layerapplied directly to said substrate, a layer having a refractive indexn_(H) applied directly to said silver layer, and another layer having arefractive index n_(H) applied directly to surface areas of saidsubstrate on which no silver layer is applied, wherein said anotherlayer has a thickness greater than a thickness of said silver layer andless than a combined thickness of said silver layer and said layerapplied directly to said silver layer.
 9. The phase annulus according toclaim 8, wherein said substrate comprises a transparent medium.
 10. Thephase annulus according to claim 9, wherein said transparent medium ischosen from the group consisting of optical glass, plastic, and crystal.11. The phase annulus according to claim 8, wherein said layers having arefractive index n_(H) comprise a dielectric material.
 12. The phaseannulus according to claim 11, wherein said dielectric material is atitanium-oxygen compound.
 13. The phase annulus according to claim 8,wherein said thin-layer system further includes a reflection-reducingcoating.
 14. The phase annulus according to claim 8, wherein the phaseshift produced can be established deliberately.
 15. A phase annulus forproducing a positive phase shift comprising a phase-shifting thin-layersystem including first and second substrates, a layer having arefractive index n₁ applied to said first substrate for producing aphase shift φ, a silver layer applied to said layer having a refractiveindex n₁, a layer having a refractive index n₂ applied to said silverlayer for protecting said silver layer, and a transparent cement fillingintermediate space between said first and second substrates such that atransparent cement matrix remains between said layer having a refractiveindex n₂ and said second substrate.
 16. The phase annulus according toclaim 15, wherein said first and second substrates comprise transparentmedia.
 17. The phase annulus according to claim 16, wherein saidtransparent media are chosen from the group consisting of optical glass,plastic, and crystal.
 18. The phase annulus according to claim 15,wherein said layer having a refractive index n₁ comprises a dielectricmaterial.
 19. The phase annulus according to claim 18, wherein saiddielectric material is a titanium-oxygen compound.
 20. The phase annulusaccording to claim 15, wherein said layer having a refractive index n₂comprises a dielectric material.
 21. The phase annulus according toclaim 20, wherein said dielectric material is a titanium-oxygencompound.
 22. The phase annulus according to claim 15, wherein saidthin-layer system further includes a reflection-reducing coating. 23.The phase annulus according to claim 15, wherein the phase shiftproduced can be established deliberately.
 24. The phase annulusaccording to claim 15, wherein said cement has a refractive indexn_(Cement) in a range from 1.56 to 1.59 and n_(Cement) is greater thann₁, whereby said positive phase shift increases with increasingthickness of said layer having refractive index n₁ relative to a phaseshift produced by a partial system consisting of said silver layer andsaid layer having refractive index n₂.
 25. The phase annulus accordingto claim 15, wherein said cement has a refractive index n_(Cement) in arange from 1.56 to 1.59 and n_(Cement) is less than n₁, whereby saidpositive phase shift decreases with increasing thickness of said layerhaving refractive index n₁ relative to a phase shift produced by apartial system consisting of said silver layer and said layer havingrefractive index n₂.